Andy Goldsworthy says his piece is "a very difficult work to unravel, yet it feels so right."

Photo: Rohan Smith, The Chronicle

Andy Goldsworthy says his piece is "a very difficult work to...

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"Tree Fall," in Building 95 on the Main Parade Ground in the Presidio, features a tree branch and ceiling covered in clay.

Photo: Rohan Smith, The Chronicle

"Tree Fall," in Building 95 on the Main Parade Ground in the...

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Andy Goldsworthy poses for a portrait with his new installation in the Presidio called 'Tree Fall.'

Photo: Rohan Smith, The Chronicle

Andy Goldsworthy poses for a portrait with his new installation in...

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"Tree Fall," a new work by arftist Andy Goldsworthy opens Oct. 19 inside the Powder Magazine in the Presidio's Main Post. The installation is constructed of a eucalyptus tree removed as part of the reconstruction of Doyle Drive and clay from surrounding land, serving as a reminder of the relationship between the Presidio's natural and built environments.

In this file photo, Andy Goldsworthy considers which logs to use for his project "Wood Line", a landscape sculpture of eucalyptus logs that snakes around the Presidio.

Photo: Skyler Reid, Special To The Chronicle

In this file photo, Andy Goldsworthy considers which logs to use...

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This 2014 photo shows "Wood Line" in progress. The Andy Goldsworthy landscape sculpture of eucalyptus logs snakes around the Presidio. Charlie Adamski, a post-war and contemporary art specialist at Christie's in San Francisco, lists this as a must-see artwork in the Bay Area. "My favorite of the four installations by the British artist in the Presidio, Wood Line is a flawless representation of art and nature in perfect harmony, " she says.

After hoisting a pencil point 100 feet into the sky and snaking a line of logs 400 yards on the ground, there was just one direction left to Andy Goldsworthy - down into the Earth.

"Tree Fall," his latest installation in the Presidio of San Francisco, is not underground in the actual sense. But you will feel that way upon entering Building 95, a windowless square of stone that stands alone in the middle of the Main Parade Ground. The ceiling has been covered in clay and is bisected by a suspended tree branch also covered in clay. The intended effect is to stand in a Hobbit hole, looking up.

"How extraordinary that that tree fits in here," says Goldsworthy, who is known as much for his droll understatement as he is for his clever use of natural materials. "A most unlikely combination, a tree in the center of a building. It's a very difficult work to unravel, yet it feels so right."

The commission by the For-Site Foundation is Goldsworthy's third in the Presidio, after "Spire" (2008) and "Wood Line" (2011). The first two used trees cleared in the Presidio reforestation process, and now he's enlisted a eucalyptus felled during the Doyle Drive demolition.

" 'Spire' represents the verticality of the forest. 'Wood Line' represents the density and the path of the forest. 'Tree Fall' is the root structure of the forest," the artist says, while here on his annual summer work-about from his home in Dumfriesshire, Scotland.

The site for "Tree Fall" was spotted by Cheryl Haines, the San Francisco gallery owner who has shown Goldsworthy's work since 1992. Haines founded For-Site in 2003 to present free temporary exhibitions in the public realm, most recently "International Orange," which drew 150,000 people to Fort Point in 2012.

As curator of "Tree Fall," Haines did the negotiations with the Presidio Trust, so that when the artist arrived in July the former storage center had been cleared out and prepared for installation.

The room, 20 by 17 feet, was originally an ammunition store and more recently held the blanks for the evening gun salute. Named the Old Stone Powder Magazine, it has never been open to the public and presented challenges that Goldsworthy does not have to deal with when working outside.

Enlisting family, volunteers

For starters, he could not touch the walls, which are 2 feet thick and date to 1863, making it one of the oldest structures in the historic fort. The project team had to build four walls inside the four walls, drop in a ceiling and pour a cement floor.

Once the stone structure was protected, Goldsworthy went to work alongside nine family members he brought with him. As captured in the famous documentary "Rivers and Tides," his style suggests that he never knows what the hell he is doing, but it is all very precise and organized.

He set up an assembly line of community volunteers to mix the clay. The primary material was dirt unearthed during excavation for the nearby officers' club restoration. The binding agent is a combination of straw and human hair, lots of it, from local salons.

The clay was hand-delivered into the dark building where Goldsworthy stood on a ladder, slapping it on in an overhead motion, hard labor for a man of 57.

"There is a lot of love and understanding with clay that has been won over many years," he says, "and you never know how it will turn out."

His hope was that the clay would dry and crack into puzzle pieces, to give the art detail and intricacy. This is a concept that Goldsworthy has been refining since he first built a clay wall, at the Haines Gallery at 49 Geary St., in 1996.

It was an experiment that had never worked for him in England, and he could not predict its life span. But the clay wall is still in the gallery, with deep cracks in it, giving the effect of a dried-up lake bottom. "California released a whole relationship with clay," he says. "I did Cheryl's wall, and when that didn't fall off, I realized that I could do a ceiling."

Natural illumination

Seventeen years later, the ceiling was done, in the Presidio. Then it was given two months to dry and cure. Now the installation, scheduled to open Saturday, will be there for a minimum of five years. The piece is unlighted. Whatever illumination there is comes through the open door.

"Look at that," he says as a wisp of fog passes in front of the sun and casts a momentary shadow on the side walls, which are painted primer white.

It has all been guesswork until this moment, when he sees it complete for the first time.

"It feels very subterranean when you come in here. We're underground, but it turns it on its head because it is above you," he says, smiling at his own logic. "I don't know. I'm not very coherent."